The evolution of wireless devices has dramatically increased the availability of voice and data services and the adoption of wireless services. This has created a significant demand for improved wireless communication terminals and in turn has complicated the design considerations for transmitters and antennas of wireless devices.
Until the advent of mobile handsets and portable wireless terminals, antenna design was often based purely on technical radio specifications such as gain, bandwidth, and polarization, for example. Demand for reduced size, increased power efficiency and the requirement to meet ever more stringent radiation emission regulation requirements have emphasized a number of additional design aspects. Specifically antenna efficiency and a reduction of user irradiation by a wireless device have become important design considerations for transmitter stages of wireless devices.
Wireless devices are subject to Specific Absorption Rate (SAR) limits in many countries to ensure that device users are not exposed to unacceptable irradiation levels. Regulations may prescribe different SAR exposure limits for different regions of the body. SAR is determined by the radiation absorbed by a person and is typically defined in terms of power absorbed per mass of tissue. SAR can depend on a number of aspects including, for example, the position and orientation of the wireless device relative to the person, the field emission characteristics of the wireless device, the transmitting power and frequency, as well as ambient elements that can interact with the radiation emitted by the wireless device, and possibly other effects such as effects related to polarization of the radiation.
As the operating frequencies for wireless devices utilizing a given communication channel are usually fixed and cannot be independently controlled, known solutions to limit SAR build upon control of one or more of the remaining aspects. Although, depending on the wireless communication protocol, transmission power and consequently user irradiation may be lower in close proximity to wireless base stations, means to limit SAR even at full transmission power are required. Designing wireless devices with sufficiently high transmitter performance and sufficiently low SAR has become increasingly challenging. For example, transmission power must be sufficiently high to provide an acceptable signal-to-noise ratio (SNR) of the transmitted signal, and sufficiently low to result in an acceptable SAR.
Known solutions to limit SAR include distancing the device or at least the transmitter from the user, reducing transmission power with decreasing distance, and controlling the near-field radiation emission characteristics. According to some solutions, wireless devices may be preconfigured with an antenna system that can emit radiation only in predetermined directions in order to direct radiation primarily away from the user. SAR can also be reduced by reducing data transmission rate in combination with certain wireless communication protocols, for example. Some known solutions employ sophisticated distance sensing schemes and/or mechanisms in order to determine the distance between the wireless device and a user that in return require wireless devices that include complex hardware and/or software.
U.S. Pat. Nos. 5,541,609, 5,815,820, 6,195,562, 6,456,856, 6,919,861, 7,146,139, 7,499,722 and United States Patent Application Publication No. 2005/0113125 describe examples of known solutions. Some known solutions for controlling SAR, however, have become ineffective in meeting today's advanced regulatory requirements while others have become increasingly complex and therefore cost-ineffective.
Another example is U.S. Pat. No. 7,212,164, which describes a radio terminal device comprising an antenna and a tilt detector for detecting the tilt angle of the radio terminal device to notify an antenna characteristic switching section that performs a predetermined reconfiguration of the antenna to adjust, for example, antenna sensitivity and gain on the basis of the tilt angle. The patent, however, is concerned with effects of the tilt on antenna reception and fails to consider user irradiation.
Therefore there is a need for a solution that overcomes at least one of the deficiencies in the art.
This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.